A trope is a recurring feature of a genre or medium, a plot device is a story point which exists solely to move things along… I think.
Plot devices and tropes oft overlap, not all tropes or plot devices are automatically bad. Much online film criticism extends as far as ‘I have noticed something, and so the film is bad’ and it irks me. The noticing of plot devices and/or tropes can be a fun and affectionate exercise.